A missile guidance system relies upon the electromagnetic signature of a target to differentiate the target from the background terrain and to locate the position of the target. An object in the field of view of the missile guidance system radiates, or emits, electromagnetic energy and also reflects electromagnetic energy. The combined emission and reflection characteristics of the object determine its electromagnetic signature.
The on-going development of millimeter wave missile seekers has created a need for increasingly more sophisticated facilities for testing the increased capabilities of each succeeding generation of missile seeking devices. It is desirable to test the target response of a new seeker design by simulating in a laboratory the electromagnetic signature of a missile seeker target and its environment. Cost and convenience factors make laboratory testing preferable to on-site testing of the seeker under actual flight conditions.
It is also desirable to be able to test with the same testing facilities the two types of missile seeker guidance systems which have evolved. Passive millimeter wave seekers differentiate a target from surrounding background by detecting differences in radiometric temperature (emissivity differences in the millimeter wave portion of the electromagnetic spectrum). Passive millimeter wave seekers are dependent upon the emissivity/reflectivity characteristic contrast between the target and the target background, and require target signature simulators having variable emissivity/reflectivity capabilities. Active millimeter wave seekers use radar techniques for locating the position of their targets. Active seekers transmit electromagnetic energy and detect the reflection of that energy from the target and the background surrounding the target. Thus, seekers employing the active mode require target signature simulators having variable reflectivity characteristics.
Development of millimeter wave seekers capable of both active and passive modes of detection has increased the desirability of a target signature simulator capable of providing both active and passive electromagnetic target signatures.
An anechoic radar simulator uses a chamber with anechoic material lining the walls. The anechoic simulator is capable of providing real-time target simulations for evaluating active mode missile seekers. However, the anechoic radar simulator cannot provide real-time target signature simulations for passive millimeter wave seekers because the high emissivity characteristics of the anechoic material on the chamber walls cause high radiometric temperatures, which mask the passive seeker under evaluation. Thus, anechoic chambers are not useful for evaluating a passive/active (dual mode) millimeter wave seeker without modification of the seeker to allow for the chamber temperature effects. It is not feasible to install such modifications in some seekers, because of the particular design of the seeker. Moreover, such modifications reduce the credibility of the resulting test data. Ideally, the simulator facility should be capable of accommodating any seeker without requiring modification of the seeker.
A target signature simulator array adapted for active mode missile seeker target tracking evaluations is disclosed in U.S. Pat. No. 4,467,323 issued to Drake et al, which is assigned to the assignee of the present invention, and which is incorporated herein by reference. As in all active mode simulators, a simulator transmitter is required to enable the range finding function of the active mode seeker to be evaluated. The Drake et al simulator uses a cluster of active antenna elements, a millimeter wave spectrum analyzer, a computer and a target control circuit to receive active mode seeker signals and to generate time-delayed simulated target response signals, the latter being received by the seeker under evaluation.
A simulation facility for dynamically testing guidance systems which use radio frequencies is disclosed in U.S. Pat. No. 4,106,345 issued to Saunders et al on Aug. 15, 1978, and which is incorporated herein by reference. This patent also is assigned to the assignee of the present invention. The Saunders et al simulation facility is designed for testing passive seekers in the radiometric frequency band comprising generally the millimeter and microwave frequency ranges (18 GHz to 350 GHz). Included in this facility is a target signature simulator array comprised of dynamically controllable noise sources which, when energized, emit energy at radiometric frequencies to simulate the radiometric appearance of terrain and selected objects and targets. Calibration of these noise sources is a difficult task, because the frequency band of the emissions varies with the power output of the emission. The array of noise sources is located inside a chamber having interior walls lined with a reflective material of very low emissivity coefficient, such as aluminum foil.
Simulator apparatus which attempt to combine simulator arrays capable of testing active mode seekers, like the Drake et al array, and a chamber used in testing passive seekers, like the Saunders et al chamber, in order to obtain a simulator facility capable of providing a dual mode simulation of the electromagnetic signature of a target and its surrounding terrain, have met with only limited success.
Active mode seekers may transmit continuously or in pulses. It is possible to test an active mode pulse seeker by placing an active mode target signature simulator array, as in Drake et al, inside a radiometrically cold simulation chamber, like the chamber of the Saunders et al facility. However, the fully reflecting walls of the radiometrically cold chamber create a problem associated with echoing signals, whether they be echoes from signals emitted by the seeker or signals emitted by the simulator apparatus. Drake et al. teaches that steps can be taken to ameliorate, but not eliminate entirely, the echo problem. These steps include preferred orientations of the chamber walls relative to the seeker and the signal transmitting apparatus of the simulator and provision for reflection baffling within the radiometrically cold chamber. However, some echo always remains to introduce secondary effects into the test data. Moreover, the different apparatus in Drake et al. require expensive millimeter wave components and are capable only of point-source active targets. An extended area scene would require a target signature simulator array of Drake et al active antenna clusters. Provision of such an array is cost prohibitive.
To date it has proved beyond the state of the art to eliminate the echo problem attendant testing of active mode seekers employing a continuous transmission signal in a radiometrically cold wall chamber, such as found in Saunders et al.